The present invention relates to a power conversion system, and more particularly to a plural inverter system for converting input DC into controlled output AC wherein both the controlled output AC and a parameter of the input DC are sensed for controlling the controlled output AC.
Copending patent application, having Ser. No. 454,434 filed on Dec. 20, 1989 and assigned to the same assignee as the present application, discloses a conversion system for providing a stepped output utilizing plural inverters. The system shown therein responds to the output AC for controlling the phase angle between the inverter outputs in order to control the output AC.
Power conversion systems are commonly used in many applications to convert a variable DC input into controlled AC output. For example, copending application Ser. No. 07/911,542 filed on Jul. 9, 1992 shows an inverter for use with systems, such as those on aircraft, where the input DC can vary 2:1 or more. If desired, the control AC output is designed to have constant voltage in order to supply loads with electrical power. The inverter used in such systems may be of the stepped waveform type. An inverter of this type is disclosed in U.S. Pat. No. 3,775,662 to Compoly et al. Such a stepped-waveform inverter generates an output having a stepped shape that generally approximates a sine wave. The purpose of approximating a sine wave is to reduce the size of the filters, provided at the output of the inverter, that are used to remove unwanted harmonics. While previous stepped-waveform inverters may accomplish such goals, efficient and effective methods of control of such inverters have heretofore been lacking.
Prior application having U.S. Ser. No. 454,434 filed on Dec. 20, 1989, discloses a power conversion system utilizing a plurality of subinverters each inverting a DC input into AC output for supply to a summing transformer which sums the outputs from the subinverters to provide AC power having a stepped waveform. The system regulates the AC power by controlling a phase angle between the waveforms produced by at least some of the subinverters.
If the input DC varies, the variation in DC must pass through the system and vary the output AC in order for corrective action to be taken by the inverter control system. The feedback loop which senses the output AC and provides appropriate correction is slow, because any changes or transients in the input DC voltage must pass through the inverter, the control, and any associated filters, each having time constants which slow up the response.